KR20200013726A - Hot press member, its manufacturing method and cold rolled steel sheet for hot press, and its manufacturing method - Google Patents

Abstract

After adjusting the component composition appropriately for the hot press member, the microstructure is formed of cementite having an austenite average grain size of 8 µm or less, a volume fraction of martensite of 90% or more, and a particle diameter of 0.05 µm or more. Ti-based precipitates having a particle size of less than 0.10 μm in a range from the surface of the member to an average of 200 μm ^{2 in} a cross section parallel to the thickness direction of the substrate, and having a particle diameter of less than 0.10 μm in the plate thickness direction from the member surface The presence of 10 or more on average per 100 μm ² not only has a very high tensile strength of TS: 1780 MPa or more after hot pressing, but also combines excellent delayed fracture properties and cross tensile strength after high resistance spot welding.

Description

Hot press member, its manufacturing method and cold rolled steel sheet for hot press, and its manufacturing method

TECHNICAL FIELD The present invention relates to a hot press member, a method for manufacturing the same, a cold rolled steel sheet for a hot press, and a method for manufacturing the same. In particular, the hot press member is intended to improve delayed fracture resistance and resistance spot weldability.

 In the present invention, the hot press member refers to a member obtained by hot pressing a cold rolled steel sheet having quenchability to increase its strength.

The cold rolled steel sheet of the present invention is not only a general cold rolled steel sheet, but also a hot dip galvanized cold rolled steel sheet (including an alloyed hot dip galvanized cold rolled steel sheet) and an electro galvanized cold rolled steel sheet (including an electric zinc nickel alloy plated cold rolled steel sheet), Aluminum plated cold rolled steel sheet;

In recent years, the regulation of CO ₂ emission has been tightened due to the increase of environmental problems, and in the automobile field, the weight reduction of the vehicle body for improving fuel efficiency has been a problem. Therefore, the thinning by the application of the high strength steel plate to an automotive component is advanced, and the application of the steel plate whose tensile strength TS is 1780 Mpa or more is examined.

High strength steel sheets used for structural members and reinforcing members of automobiles are required to be excellent in formability. However, since steel sheets of TS: 1780 MPa or more have low ductility, cracks occur during cold press molding or large spring back occurs due to high yield strength, so that high dimensional accuracy is not obtained after cold press molding. In addition, since the residual stress remains in the steel sheet after cold press forming, delayed fracture (hydrogen embrittlement) is feared by hydrogen invading from the use environment.

In such a situation, as a method of obtaining high strength, press molding by hot press (also called hot stamp, die quench, press quench, etc.) has attracted attention in recent years. With hot press, after heating a steel plate to the temperature range of an austenite single phase, it shape | molds (processes) at high temperature, it enables shaping | molding with high dimensional precision, and high strength is possible by quenching by cooling after shaping | molding. One molding method. Moreover, in this hot press, since the residual stress after press molding falls compared with a cold press, delayed fracture characteristic is also improved.

However, many of the automobile assembly processes are assembled by resistance spot welding. However, in order to maintain the rigidity of the entire automobile body at that time, stress is applied to the member after hot pressing, so that there is no fear of delayed fracture after press molding. . Therefore, it is necessary to improve the delayed fracture characteristic of the member after hot press.

Moreover, in order to ensure the tensile strength of 1780 MPa or more, it is necessary to contain a large amount of alloying elements (for example, C, etc.), but there is a concern that the cross tensile strength (CTS) of the joint after the resistance spot welding is remarkably lowered. .

Conventionally, some means for improving the delayed fracture resistance property after hot press have been reported.

For example, Patent Literature 1 discloses a technique for improving delayed fracture resistance by controlling the amount of precipitation of alloy carbonitride and cementite.

In addition, Patent Literature 2 discloses a technique for improving delayed fracture resistance by forming residual austenite after hot pressing.

Japanese Unexamined Patent Publication No. 2015-113500 Japanese Unexamined Patent Publication No. 2014-122398

However, in the Ti-based carbides described in Patent Document 1, it is insufficient to achieve miniaturization of the former austenite grain size, and also has a sufficient delayed fracture-destructive characteristic since the function as a trap site of hydrogen penetrating from the surface is insufficient. Can not. Moreover, it cannot be said that the cross tensile strength after resistance spot welding is ensured.

In the technique of Patent Literature 2, the retained austenite can be a trap site for hydrogen, but when there is a high austenite retained austenite, the hardness distribution becomes large at the heat affected zone (HAZ) after the resistance spot welding, and the cross tensile strength Falls.

As described above, it is considered difficult to improve both the delayed fracture resistance of the hot press member of TS: 1780 MPa or more and the cross tensile strength after resistance spot welding, and no hot press member having these characteristics has been developed. It is a fact.

Therefore, the present inventors have made diligent studies in view of the above circumstances, and as a result, in order to improve both the delayed fracture resistance of the hot press member and the cross tensile strength after resistance spot welding, the microstructure of the member is a fine Ti system. It is found that it is effective to disperse the precipitate in the surface layer of the member and to precipitate cementite in the martensite as a trap site for hydrogen, thereby improving both the excellent delayed fracture resistance and the cross tensile strength after resistance spot welding. Paid.

Specifically, by dispersing the fine Ti-based precipitates on the surface of the steel sheet, the former austenite average grain size is refined, and the Ti-based precipitate functions as a trap site for hydrogen that penetrates from the surface with corrosion, resulting in delayed fracture characteristics. Is improved.

 In addition, since the Ti-based precipitate refines the microstructure of the heat affected zone HAZ even after the temperature is raised by resistance spot welding, the toughness to the stress applied to the nugget end is improved, and the hardness decrease due to HAZ softening is also achieved. Since it is suppressed, the cross tensile strength improves.

In addition, when cementite is dispersed in the martensite of the microstructure of the member, the cementite becomes a trap site for hydrogen, contributing to the improvement of the delayed fracture characteristic. In addition, the presence of cementite having a particle diameter of 0.05 µm or more prevents the cementite from completely melting in the HAZ softening portion after the resistance spot welding, and thus reduces the solid solution C. Thus, it has been found that the toughness can be ensured and the cross tensile strength is improved.

Regarding the steel component of the member, when the amount of Mn is increased, the phase transformation behavior in the nugget is changed, and only a part of the δ phase transformation occurs, or the δ phase transformation does not occur, but changes from the liquid to the γ phase. For this reason, Mn and P remain segregated, segregation of the nugget end after welding ends is remarkable, and the segregation point is easily broken, making it difficult to secure the cross tensile strength, so the amount of C, Mn, and P added It is desirable to consider. If the amount of Mn is small, the quenchability at the time of hot press cannot be secured, and the tensile strength after the hot press becomes difficult. However, the quenchability can be secured by increasing Cr or Mo. Moreover, Cr and Mo are useful because they have little influence on the transformation behavior in the nugget described above. In addition, Ti affects securing delayed fracture resistance and cross tensile strength as described above.

Therefore, it is preferable to consider the ratio of the addition amount of C, Mn, P and the addition amount of Cr, Mo, and Ti to improve the delayed fracture resistance and the cross tensile strength.

This invention is based on the said knowledge.

That is, the summary structure of this invention is as follows.

1. The steel chemical component of the member is, by mass%, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.1% or more and 2.4% or less, P: 0.05% or less, S: 0.005% or less, Al : 0.01% or more, 0.50% or less, N: 0.010% or less, Ti: 0.005% or more and 0.15% or less, and Mo or 0.50% or less and Cr: 0.50% or less The balance consists of Fe and inevitable impurities,

In the microstructure of the member, cementite having an austenite average grain size of 8 µm or less, a volume fraction of martensite of 90% or more, and a particle diameter of 0.05 µm or more was averaged per 200 µm ^{2 of} a cross section parallel to the thickness direction of the member. More than,

Further, at least 10 Ti-based precipitates having a particle diameter of less than 0.10 μm in the range from the surface of the member to 100 μm in average per 100 μm ^{2 of} the cross section parallel to the thickness direction of the member and having a tensile strength of at least 1780 MPa were hot Press member.

2. The said member is the mass% further Nb: 0.15% or less, B: 0.0050% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less , V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less and W : The hot press member according to the above 1 containing one or two or more selected from 0.10% or less.

3. The hot press member according to the above 1 or 2, in which C, P, Mn, Cr, Mo, and Ti satisfy the following formula (1) among the strong chemical components of the member.

(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)

Here, [M] is content (mass%) of M element, and when it does not contain element [M], it calculates as 0.

4. The hot press member according to any one of 1 to 3 above, wherein the surface layer of the member has an Al-based plating layer or a Zn-based plating layer.

5. The chemical composition of the steel sheet is, by mass%, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.1% or more and 2.4% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more, 0.50% or less, N: 0.010% or less, Ti: 0.005% or more, 0.15% or less, and further, one or two kinds selected from Mo: 0.50% or less and Cr: 0.50% or less, Wealth consists of Fe and inevitable impurities,

The microstructure of the steel sheet contains a martensite having an average grain size of 4 µm or less in a range of 5 to 45% by volume ratio, and a Ti system having a particle diameter of less than 0.10 µm in a range from the surface of the steel sheet to 100 µm in the plate thickness direction. The cold-rolled steel sheet for hot press in which 15 or more precipitates exist in an average per 100 micrometer <^2> of cross sections parallel to the plate thickness direction of a steel plate.

6. The steel sheet is, in mass%, further Nb: 0.15% or less, B: 0.0050% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less , V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less and W : The cold rolled steel sheet for hot pressing according to the above 5, containing one kind or two or more kinds selected from 0.10% or less.

 7. Among the chemical components of the steel sheet, in particular, the cold rolled steel sheet for hot pressing according to 5 or 6, wherein C, P, Mn, Cr, Mo, and Ti satisfy the following formula (1).

(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)

Here, [M] is content (mass%) of M element, and when it does not contain element [M], it calculates as 0.

8. The cold rolled steel sheet for hot pressing according to any one of 5 to 7, wherein the steel sheet has an Al-based plating layer or a Zn-based plating layer on its surface.

9. A method of manufacturing the cold rolled steel sheet for hot pressing according to the above 5,

In mass%, C: 0.28% or more but less than 0.42%, Si: 1.5% or less, Mn: 1.1% or more and 2.4% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.50% or less, N : 0.010% or less, Ti: 0.005% or more, 0.15% or less, and 1 or 2 types selected from Mo: 0.50% or less and Cr: 0.50% or less, and remainder consists of Fe and an unavoidable impurity. Molten steel is continuously cast to a slab, and the slab is cooled to 650 ° C. within 6 h,

Then, it reheats and hot-rolls on the conditions that the reduction ratio of the final pass of finishing rolling is 12% or more, the reduction ratio of the pass just before the final path is 15% or more, and the finishing rolling end temperature is 860-950 degreeC. ,

After the said hot rolling, the 1st average cooling rate to a cooling stop temperature shall be 70 degreeC / s or more, and the primary cooling which cools to the cooling stop temperature of 700 degrees C or less is performed,

After the said primary cooling, the 2nd average cooling rate to winding temperature shall be 5-50 degreeC / s, the secondary cooling which winds up at the winding temperature of 450 degrees C or less,

Next, after pickling the wound hot rolled steel sheet and performing cold rolling, it heats to the temperature range of 700-830 degreeC at the average temperature increase rate of 5-20 degreeC / s, and the annealing which cracks for 15 to 600 second in the temperature range is carried out. Conduct,

After the said cracking process, the 3rd average cooling rate shall be 5 degrees C / s or more, and the 3rd cooling which cools to the cooling stop temperature of 600 degrees C or less is performed, The manufacturing method of the cold rolled steel sheet for hot presses.

10. The molten steel is, in mass%, further Nb: 0.15% or less, B: 0.0050% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less , V: 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less and W : The manufacturing method of the said hot-rolled cold rolled steel sheet as described in said 9 containing 1 type (s) or 2 or more types chosen from 0.10% or less.

11. Among the chemical components of molten steel, in particular, the manufacturing method of the hot-rolled cold rolled steel sheet as described in said 9 or 10 in which C, P, Mn, Cr, Mo, and Ti satisfy following formula (1).

(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)

Here, [M] is content (mass%) of M element, and when it does not contain element [M], it calculates as 0.

12. The method for producing a cold-rolled steel sheet for hot pressing according to any one of 9 to 11, wherein after the tertiary cooling, an Al-based plating treatment or a Zn-based plating treatment is further applied to the steel sheet surface.

13. The method for producing a hot press member to the cold-rolled steel sheet for hot press forming according to any one of the above items 5 to 8, and then subjected to heating, hot press in the temperature range of Ac ₃ transformation point ~ 1000 ℃.

According to the present invention, a hot press member having very high tensile strength after hot pressing and having excellent delayed fracture resistance and cross tensile strength after high resistance spot welding can be obtained. For example, the tensile strength is 1780 MPa or more, cracks do not occur even after hydrochloric acid immersion, and cross tensile strength after resistance spot welding shows 5 kN or more (preferably 6.5 kN or more) under the condition of a nugget diameter of 6.1 mm. A hot press member excellent in delayed fracture characteristics and cross tensile strength after resistance spot welding can be obtained stably.

 Moreover, according to this invention, even if it is a hot press condition with a big deviation at the time of a heating, the hot press member with stable characteristic can be obtained.

Hereinafter, the present invention will be described in detail.

 First, the microstructure of the hot press member and the cold rolled steel sheet for hot press of the present invention will be described in detail.

[Microstructure of Hot Press Member]

The microstructure of the hot press member has an average grain size of 200 μm ² of cementite having an austenite average grain size of 8 μm or less, a volume fraction of martensite of 90% or more, and a particle diameter of 0.05 μm or more parallel to the thickness direction of the member. Micro-precipitates with 10 or more Ti-based precipitates having a particle size of less than 0.10 μm in the range from the surface of the member to 100 μm in the plate thickness direction, on the average per 100 μm ^{2 of} the cross section parallel to the thickness direction of the member It is organized.

When the austenite average grain size exceeds 8 µm, the delayed fracture resistance deteriorates, so the upper limit thereof is 8 µm. Preferably it is 7 micrometers or less, More preferably, it is 6.5 micrometers or less.

Moreover, when the volume ratio of martensite is less than 90%, it will become difficult to achieve tensile strength: 1780 Mpa or more. Therefore, the volume ratio of martensite is made into 90% or more. Preferably it is 93% or more, More preferably, it is 95% or more. 100% may be sufficient.

As a remainder structure, ferrite, bainite, pearlite, etc. are considered, but if they are 4% or less in total, there is no problem.

In the cross section parallel to the thickness direction of the member after hot pressing, it is necessary to have 10 or more cementite having a particle diameter of 0.05 µm or more on average per 200 µm ^{2 of} the cross section. Preferably it is 20 or more. As the cementite is precipitated in this manner, it becomes a trap site for hydrogen, contributing to the improvement of the delayed fracture resistance, and in the HAZ softening section after the resistance spot welding, the cementite does not melt completely and the solid solution C decreases due to the resistance spot welding. The toughness of the HAZ softened portion after welding is improved, and as a result, the cross tensile strength is improved. On the other hand, when the particle size of cementite is less than 0.05 µm or the particle size is 0.05 µm or more, if the number is less than 10 on average, the delayed fracture resistance and the cross tensile strength after resistance spot welding deteriorate. Moreover, there is no restriction | limiting in particular about the upper limit of the particle diameter of cementite, It is preferable to set it as 1 micrometer or less.

Here, even if cementite having a particle diameter of less than 0.05 µm exists, if 10 or more cementite having a particle diameter of 0.05 µm or more exists on average, desired characteristics can be secured. In addition, there is no restriction | limiting in particular about the cross section parallel to the thickness direction of the member to measure, Anywhere may be sufficient.

Moreover, it is necessary to exist ten or more Ti-type precipitates whose particle diameter is less than 0.10 micrometer in the range from the member surface to 100 micrometers in average per 100 micrometer <^2> of cross sections parallel to the thickness direction of a member. Preferably it is 15 or more. By dispersing the Ti-based precipitate having a particle size of less than 0.10 mu m in the surface layer of the member as described above, the delayed fracture resistance and the cross tensile strength after resistance spot welding are improved. On the other hand, if the Ti-based precipitate is 0.10 µm or more, or the particle size is less than 0.10 µm, the number thereof is less than 10 on average, the improvement of sufficient delayed fracture resistance and cross tensile strength after resistance spot welding cannot be expected. Here, as Ti type | system | group precipitate, TiC, TiN, Ti (C, N) etc. are mentioned, for example. In addition, even if there are Ti-based precipitates having a particle size of 0.10 µm or more, if 10 or more Ti-based precipitates having a particle size of less than 0.10 µm are averaged, desired characteristics can be secured.

In addition, there is no restriction | limiting in particular about the cross section parallel to the thickness direction of the member to measure, Anywhere may be sufficient. In addition, the minimum of the particle diameter of the target Ti-based precipitate is made 0.005 µm.

[Microstructure of Cold Rolled Steel Sheet for Hot Press]

In order to obtain desired characteristics as a hot press member, it is important to control the microstructure of the cold rolled steel sheet for hot press. That is, as a microstructure of the cold rolled steel sheet for hot pressing, the martensite having an average grain size of 4 µm or less is contained in the range of 5 to 45% by volume ratio, and in the range of up to 100 µm in the sheet thickness direction from the surface layer. 15 or more Ti-based precipitates having a thickness of less than 0.10 µm are present on average per 100 µm ² of cross section parallel to the plate thickness direction of the steel sheet.

In the cold rolled steel sheet for hot pressing, when the average grain size of martensite exceeds 4 µm, the concentration distribution of C and Mn changes during hot pressing, and thus the dispersion state of the desired cementite is not obtained. And cross tensile strength decreases.

 In addition, even if the volume fraction of martensite is less than 5% or more than 45%, the desired dispersed state of cementite is not obtained in the same manner, so that the delayed fracture resistance and the cross tensile strength decrease. Preferably it is 10% or more and 40% or less of range.

In addition, since Ti-based precipitates coarsened by hot pressing exist, Ti-based precipitates having a particle size of less than 0.10 µm are hot in an average of less than 15 per 100 µm ² in a cross section parallel to the plate thickness direction of the cold rolled steel sheet. Since the desired distribution form of the Ti-based precipitate is not obtained after pressing, there is a fear that the delayed fracture resistance and the cross tensile strength after the resistance spot welding are lowered. Therefore, as a cold rolled steel sheet before hot press, 15 or more Ti-type precipitates with a particle size of less than 0.10 micrometer have an average per 100 micrometer <^2> in cross section parallel to the plate thickness direction of a steel plate in the range from the surface layer to 100 micrometers in the plate thickness direction. It is done. Preferably it is 20 or more. In addition, there is no restriction | limiting in particular about the cross section parallel to the plate thickness direction of the steel plate to measure, What is called a C cross section, L cross section may be sufficient, or any may be sufficient as it.

In addition, in order to obtain a desired old austenite particle size after hot pressing, as a microstructure of the cold rolled steel sheet before hot pressing, a ferrite having an average aspect ratio of 3.0 or less and an average grain size of 7 μm or less is contained in a volume ratio of 20% or more. It is more preferable. The upper limit with preferable volume ratio is 80%. This is because C and Mn are concentrated in hard phases other than ferrite, and the desired grain size of the old austenite is not obtained after hot pressing.

As the remainder structure, bainite, pearlite and the like can be considered, but if these are 25% or less in total, there is no problem.

In the cold rolled steel sheet for hot press, the requirement that the average grain size of martensite of 4 µm or less is in the range of 5 to 45% by volume ratio is mainly used in the continuous casting, hot rolling, and annealing processes during the manufacturing process of the cold rolled steel sheet described later. In addition, the requirement that 15 or more Ti-based precipitates having a particle diameter of less than 0.10 µm in the range from the surface of the steel sheet to the thickness of 100 µm in average per 100 µm ^{2 of} the cross section parallel to the sheet thickness direction of the steel sheet is mainly continuous casting. , By hot rolling and annealing processes.

Next, the appropriate component composition range of the hot press member and the cold rolled steel sheet for hot press of this invention is demonstrated. In addition, "%" display with respect to a component means "mass%."

C: 0.28% or more and less than 0.42%

C is an effective element for increasing the strength of steel, and is an important element for enhancing the strength of steel by reinforcing martensite after hot pressing. However, when the content of C is less than 0.28%, the hardness of martensite after hot pressing is insufficient, so that tensile strength: 1780 MPa or more is not obtained. Preferable amount of C is 0.30% or more. On the other hand, when C is added 0.42% or more, the hardness after resistance spot welding becomes hard, toughness falls, and cross tensile strength falls. Therefore, the amount of C is made into less than 0.40%. Preferably it is less than 0.39%.

Si: 1.5% or less

Si is an element effective in solidifying and strengthening ferrite. However, since excessive addition of Si reduces toughness at the time of resistance spot welding and cross tensile strength deteriorates, the content is made into 1.5% or less. Preferably it is 1.2% or less. In addition, although the minimum of Si is not specifically defined, since extremely low Siization raises cost, it is preferable to set it as 0.005%.

Mn: 1.1% or more and 2.4% or less

Since Mn improves the hardenability at the time of hot press, Mn is an element which contributes to formation of martensite after hot press, ie, high strength. In order to acquire the effect, it is necessary to make Mn amount 1.1% or more. Preferably it is 1.3% or more. On the other hand, when Mn is excessively contained, since P segregates after the resistance spot welding and the cross tensile strength decreases, the amount of Mn is made 2.4% or less. Preferably it is 2.2% or less, More preferably, it is less than 2.0%.

P: 0.05% or less

P contributes to high strength due to solid solution strengthening, but when excessively added, segregation to grain boundaries becomes significant and embrittlement of grain boundaries. Thus, cross tensile strength after resistance spot welding decreases, so that the P content is 0.05% or less. do. Preferably it is 0.02% or less. In addition, although the minimum of P is not specifically defined, since ultra-low P formation raises steelmaking cost, it is preferable to set it as 0.0005%.

S: 0.005% or less

In the case where the content of S is large, many sulfides such as MnS are generated, and the inclusions start at the time of hydrogen intrusion and cause cracks to occur, thereby degrading the delayed fracture characteristics. Therefore, the upper limit of S content is made into 0.005%. Preferably it is 0.0045% or less. In addition, although the minimum of S is not specifically prescribed | regulated, since ultra-low S formation raises steelmaking cost similarly to P, it is preferable to set it as 0.0002%.

Al: 0.01% or more and 0.50% or less

Al is an element necessary for deoxidation, and in order to acquire this effect, it is necessary to contain Al 0.01% or more. On the other hand, since an effect is saturated even if it contains Al exceeding 0.50%, Al amount may be 0.50% or less. Preferably it is 0.40% or less.

N: 0.010% or less

Since N forms coarse nitride and degrades delayed fracture characteristic, it is necessary to suppress content. In particular, when N amount exceeds 0.010%, this tendency becomes remarkable, so the N content is made 0.010% or less. Preferably it is 0.008% or less.

Ti: 0.005% or more and 0.15% or less

Ti is an element which contributes to an increase in strength by forming fine carbonitrides. In addition, in the present invention, by dispersing fine Ti precipitate in the member surface layer, it contributes to miniaturization of the microstructure grains of the member after the hydrogen trap site and the spot welding, and improves the delayed fracture characteristics and the cross tensile strength after the resistance spot welding. It works. In order to exhibit such an effect, it is necessary to contain Ti 0.005% or more. Preferably it is 0.010% or more. On the other hand, when Ti is added in a large amount, since the elongation after hot press falls remarkably, Ti content is made into 0.15% or less. Preferably it is 0.12% or less.

Mo: 0.50% or less

Since Mo improves the hardenability at the time of hot press, Mo is an element which contributes to formation of martensite after hot press, ie, high strength. In order to acquire the effect, it is preferable to contain Mo 0.005% or more. More preferably, it is 0.01% or more. On the other hand, even if Mo is added in a large amount, the above effect is saturated, rather it causes an increase in cost and deteriorates the chemical conversion processability. Therefore, the Mo content is made 0.50% or less.

Cr: 0.50% or less

Cr is also an element that contributes to the formation of martensite after hot pressing, that is, to high strength, since the hardenability during hot pressing is increased similarly to Mo. In order to acquire the effect, it is preferable to contain 0.005% or more. More preferably, it is 0.01% or more. On the other hand, even if Cr is added in a large amount, the above effect is saturated, and since the plating property is deteriorated because the surface oxide is formed, the Cr content is made 0.50% or less.

Moreover, in this invention, it is preferable to satisfy following formula (1) especially about C, P, Mn, Cr, Mo, and Ti among a component.

(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)

Here, [M] is content of M element (mass%)

The above-described formula serves as an index for securing delayed fracture characteristics and cross tensile strength, and when the value of the left side exceeds 30.5, it is difficult to have delayed fracture characteristics and cross tensile strength. have.

In addition, in the present invention, the following components may be appropriately contained.

Nb: 0.15% or less

Nb is an element which contributes to an increase in strength by forming fine carbonitrides. In addition, in the present invention, in addition to being a trap site for hydrogen, the fine Nb-based precipitates further refine the austenite grain size during hot pressing, thereby contributing to the improvement of delayed fracture characteristics and cross tensile strength after resistance spot welding. It is an element to say. In order to exhibit such an effect, it is preferable to contain Nb 0.005% or more. On the other hand, even if a large amount of Nb is added, the above effect is saturated, and rather, the cost is increased. Therefore, the Nb content is preferably 0.15% or less. More preferably, it is 0.12% or less, More preferably, it is 0.10% or less.

B: 0.0050% or less

B is an element that contributes to the formation of martensite after hot pressing, that is, to high strength, since the hardenability during hot pressing is improved. Moreover, since segregation at grain boundaries improves grain boundary strength, it is effective for delayed fracture characteristics. In order to express such an effect, it is preferable to contain B 0.0002% or more. However, since excessive addition of B degrades toughness and lowers the cross tensile strength after resistance spot welding, it is preferable to make B content into 0.0050% or less. More preferably, it is 0.0040% or less.

Sb: 0.001% or more and 0.020% or less

Sb has the effect of suppressing the decarburization layer which arises in a steel plate surface layer part after cooling a steel plate by a series of processes of a hot press after heating a steel plate before hot press. Therefore, the hardness distribution of the plate surface becomes uniform, and the delayed fracture characteristic is improved. In order to express such an effect, it is preferable to make the addition amount of Sb into 0.001% or more. On the other hand, when Sb is added exceeding 0.020%, since rolling load load will increase and productivity will fall, it is preferable to make Sb amount into 0.020% or less.

Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less

Ca, Mg, and REM control the shapes of sulfides and oxides and suppress the formation of coarse inclusions, thereby improving delayed fracture resistance. In order to express such an effect, it is preferable to add 0.0005% or more, respectively. On the other hand, since excessive addition causes an increase in inclusions and deteriorates the delayed fracture characteristics, it is preferable that the amount of each addition be 0.005% or less. Where REM is an element comprising Sc, Y and lanthanoids.

V: 0.15% or less

V is an element which contributes to an increase in strength by forming fine carbonitrides. In order to acquire such an effect, it is preferable to contain V 0.01% or more. On the other hand, since a large amount of V addition falls in toughness at the time of resistance spot welding and cross tensile strength deteriorates, it is preferable to make V addition amount 0.15% or less.

Cu: 0.50% or less

Cu not only contributes to high strength by solid solution strengthening, but also improves corrosion resistance, so that delayed fracture resistance can be improved, and therefore, Cu can be added as necessary. In order to exhibit these effects, it is preferable to contain Cu 0.05% or more. On the other hand, even if it contains Cu in excess of 0.50%, since an effect becomes saturated and surface defects resulting from Cu tend to arise, it is preferable to make Cu content into 0.50% or less.

Ni: 0.50% or less

Since Ni also improves corrosion resistance similarly to Cu, since delayed fracture resistance can be improved, it can be added as needed. Moreover, when it adds simultaneously with Cu, since there exists an effect of suppressing the surface defect which originates in Cu, it is effective at the time of Cu addition. In order to exhibit these effects, it is preferable to contain Ni 0.05% or more. However, since a large amount of Ni addition reduces toughness at the time of resistance welding and cross tensile strength falls, it is preferable to make Ni content into 0.50% or less.

Sn: 0.50% or less

Sn also improves corrosion resistance similarly to Cu and Ni, so that delayed fracture resistance can be improved, and therefore, it can be added as necessary. In order to exhibit these effects, it is preferable to contain Sn 0.05% or more. However, since a large amount of Sn adds the toughness at the time of resistance welding, and the improvement of a delayed fracture | rupture characteristic and the cross tensile strength after resistance spot welding falls, it is preferable to make Sn content into 0.50% or less.

Zn: 0.10% or less

Since Zn improves the hardenability at the time of hot press, Zn is an element which contributes to formation of martensite after hot press, ie, high strength. In order to exhibit these effects, it is preferable to contain Zn 0.005% or more. However, since a large amount of Zn addition reduces the toughness at the time of resistance welding and cross tensile strength falls, it is preferable to make Zn content 0.10% or less.

Co: 0.10% or less

Co, like Cu and Ni, also improves hydrogen overvoltage and improves corrosion resistance, so that delayed fracture resistance can be improved, and thus can be added as necessary. In order to exhibit these effects, it is preferable to contain Co 0.005% or more. However, since a large amount of Co addition reduces toughness at the time of resistance welding and cross tensile strength falls, it is preferable to make Co content into 0.10% or less.

Zr: 0.10% or less

Since Zr also improves corrosion resistance similarly to Cu and Ni, since delayed fracture resistance can be improved, it can be added as needed. In order to exhibit these effects, it is preferable to contain Zr 0.005% or more. However, since a large amount of Zr addition reduces toughness at the time of resistance welding and cross tensile strength falls, it is preferable to make Zr content 0.10% or less.

Ta: 0.10% or less

Ta, like Ti, forms alloy carbides and alloy nitrides and contributes to high strength. In order to acquire the effect, it is preferable to add 0.005% or more. On the other hand, even if Ta is added in excess, the addition effect is saturated, and the alloy cost also increases. Therefore, it is preferable to make the addition amount 0.10% or less.

W: 0.10% or less

W also has the same corrosion resistance as Cu and Ni, so that the delayed fracture resistance can be improved. Therefore, W can be added as necessary. In order to exhibit these effects, it is preferable to contain W 0.005% or more. However, since a large amount of W addition falls in toughness at the time of resistance welding and a cross tensile strength falls, it is preferable to make W content into 0.10% or less.

Remainder other than what was mentioned above shall be Fe and an unavoidable impurity.

Next, the plating layer of the hot-rolled cold rolled steel sheet and hot press members of this invention is demonstrated in detail.

[Plating Layer of Cold Rolled Steel Sheet for Hot Press]

The cold rolled steel sheet for hot press of the present invention may be a cold rolled steel sheet without a plated layer. However, in order to prevent oxidation by hot press or to improve corrosion resistance, a plated layer is formed on the surface of the cold rolled steel sheet before hot press. You may give it.

In the present invention, the Al-based plating layer or the Zn-based plating layer is preferable as the plating layer provided on the surface of the hot rolled cold rolled steel sheet. By providing these plating layers to the surface of the cold rolled steel sheet for hot press, oxidation of the steel plate surface by hot press is prevented and the corrosion resistance of a hot press member improves.

As an Al type plating layer, the Al-Si plating layer formed by the hot-dip plating method is illustrated, for example. Moreover, as a Zn type plating layer, the molten Zn plating layer formed by the hot-dip plating method, the alloyed hot-dip Zn plating layer which alloyed this, the electroplated Zn plating layer formed by the electroplating method, the electric Zn-Ni alloy plating layer, etc. are illustrated, for example.

However, the Al-based plating layer or the Zn-based plating layer is not limited to the above plating layer, and in addition to Al or Zn as main components, Si, Mg, Ni, Fe, Co, Mn, Sn, Pb, Be, B, P, S, Ti Or a plating layer containing one or two or more of V, W, Mo, Sb, Cd, Nb, Cr, and Sr. The method for forming the Al-based plating layer or the Zn-based plating layer is not limited at all, and any known hot dip plating method, electroplating method, vapor deposition plating method, or the like can be applied. Moreover, the plating layer which performed the alloying process after a plating process may be sufficient as Al type plating layer or Zn type plating layer.

In the present invention, the Zn-based plating layer is more preferably a Zn-Ni alloy plating layer, in particular, to further improve the corrosion resistance of the hot press member or to prevent liquid metal brittle cracking caused by molten Zn during hot press molding. Do.

The adhesion amount of a plating layer is not specifically limited, What is necessary is just a general thing. For example, it is preferable to have a plating layer whose plating adhesion amount per single side | surface is 5-150 g / m <2>. If the coating weight is less than 5 g / m 2, it may be difficult to secure the corrosion resistance. On the other hand, if the plating adhesion amount exceeds 150 g / m 2, the plating peeling resistance may deteriorate.

[Plating Layer of Hot Press Member]

When the hot-rolled cold rolled steel sheet provided with the Al-based plating layer or the Zn-based plating layer is heated, and then hot-pressed, some or all of the plating layer components contained in the Al-based plating layer or the Zn-based plating layer are diffused into the base steel sheet and are dissolved. At the same time, a phase or intermetallic compound is produced, on the contrary, Fe, which is a base steel sheet component, is diffused in the Al-based plating layer or in the Zn-based plating layer to produce a solid solution or intermetallic compound. An oxide film containing Al is formed on the surface of the Al-based plating layer, and an oxide film containing Zn is formed on the surface of the Zn-based plating layer.

For example, when the Al-Si plating layer is heated, the plating layer changes to a plating layer mainly composed of a Fe-Al intermetallic compound containing Si. Moreover, when a molten Zn plating layer, an alloying molten Zn plating layer, an electrical Zn plating layer, etc. are heated, FeZn solid solution phase which Zn dissolved in Fe, ZnFe intermetallic compound, ZnO layer of a surface layer, etc. are formed. When the Zn-Ni alloy plating layer is heated, a solid solution layer containing Ni in which the plating layer component is dissolved in Fe, an intermetallic compound mainly composed of ZnNi, a ZnO layer on the surface layer, and the like are formed.

In the present invention, as described above, the plated layer containing Al formed by heating the cold rolled steel sheet for hot pressing provided with the Al-based plated layer is called an Al-based plated layer, and for the hot press with the Zn-based plated layer. The plating layer containing Zn formed by heating a cold rolled steel sheet shall be called a Zn type plating layer.

Next, the preferable manufacturing method of the cold rolled steel sheet for hot presses of this invention is demonstrated.

In the present invention, in the production of the cold rolled steel sheet, first, molten steel having the above-described predetermined component composition is continuously cast to form a slab, and the slab is cooled to 650 ° C within 6 h.

 Then, it reheats and hot-rolls on the conditions that the reduction ratio of the final pass of finish rolling is 12% or more, the reduction rate of the pass just before the last pass is 15% or more, and the finish rolling end temperature is 860-950 degreeC. .

After the said hot rolling, the 1st average cooling rate to a cooling stop temperature shall be 70 degreeC / s or more, and the primary cooling which cools to the cooling stop temperature of 700 degrees C or less is performed.

After the said primary cooling, the 2nd average cooling rate to winding temperature shall be 5-50 degreeC / s, and secondary cooling which winds up at the winding temperature of 450 degrees C or less is performed.

Next, after pickling the wound hot rolled steel sheet and performing cold rolling, it heats to the temperature range of 700-830 degreeC at the average temperature increase rate of 5-20 degreeC / s, and the annealing which cracks for 15 to 600 second in the temperature range is carried out. Conduct.

After the said cracking process, 3rd average cooling rate is made into 5 degreeC / s or more, and tertiary cooling which cools to the cooling stop temperature of 600 degrees C or less is performed.

Hereinafter, the above manufacturing process will be described in detail for each process.

[Continuous casting process]

In the present invention, the slab is first cast by the continuous casting method. It is because the continuous casting method becomes a premise from the subject of this invention, and its production efficiency is high compared with the casting casting method. As a continuous casting machine, a vertical bending type is preferable. This is because the vertical bending type is excellent in the balance between the equipment cost and the surface quality, and the effect of suppressing the surface crack is remarkably exhibited.

[Slab cooling process]

After slab through the continuous casting, the mixture is cooled to 650 ° C. within 6 h. After continuous casting, cooling to 650 ° C for more than 6 h results in remarkable segregation such as Mn and coarsening of crystal grains. Therefore, a desired grain size cannot be obtained in the steel sheet after cold rolling and the member after hot pressing. . In addition, since Ti-based precipitates are coarsened and do not re-dissolve before hot rolling, coarse Ti-based precipitates exist, the desired dispersion state of the Ti-based precipitates is not obtained in the cold rolled steel sheet and the member after hot pressing. If the hot rolling start temperature is increased or the time is increased, the coarsened Ti-based precipitate can be dissolved. On the other hand, the grain size becomes large, and the desired grain size is not obtained in the steel sheet after cold rolling and the member after hot pressing.

Therefore, this slab cooling process is an important manufacturing process in this invention, and cooling of the steel slab after continuous casting shall be within 6 h to 650 degreeC. Preferably it cools to within 5 h to 650 degreeC, More preferably, it cools within 4 h to 650 degreeC. Moreover, if it cools to 650 degreeC, after cooling to room temperature continuously after that, you may reheat and hot-roll, you may reheat as it is on a whole, and you may provide for hot rolling.

[Heating process]

It is preferable that the steel slab which is a raw material starts hot rolling after casting, and starts hot rolling at 1150-1270 degreeC, without reheating, or after reheating at 1150-1270 degreeC. Preferable conditions of hot rolling are hot rolling of a steel slab at the hot rolling start temperature of 1150-1270 degreeC first.

[Hot rolling process]

Reduction rate of the final pass of finishing rolling: 12% or more

A reduction ratio of the final pass of the finish rolling to 12% or more introduces a large number of shear zones into the austenite grain, increases the nucleation site during ferrite transformation after hot rolling, and makes the microstructure grains of the hot rolled sheet finer. And from the viewpoint of eliminating the Mn band. It is also effective for miniaturization of microstructure crystal grains on the surface layer. The suitable rolling reduction of the final pass of finish rolling is 13% or more. Moreover, although the upper limit of this rolling reduction rate is not specifically limited, 30% or less is preferable because there exists a possibility that the plate thickness fluctuations in the width direction of a steel plate may increase, and a delayed fracture-breaking characteristic may deteriorate when hot-rolled load load increases.

Reduction rate of the pass just before the final pass of finishing rolling: 15% or more

The reduction ratio of the pass just before the final pass is 15% or more, so that the strain accumulation effect is higher, a large number of shear zones are introduced into the austenite grain, the nucleation site of the ferrite transformation is further increased, and the microstructure grain of the hot rolled sheet is more. It is necessary from the viewpoint of miniaturization and eliminating the Mn band. The suitable rolling reduction rate of the pass just before the final pass of finish rolling is 18% or more. Moreover, although the upper limit of this rolling reduction rate is not specifically limited, 30% or less is preferable because when the hot-rolled load load increases, plate | board thickness fluctuations in the width direction of a steel plate become large, and deterioration of delayed fracture characteristic is feared.

Finish rolling finish temperature: 860 to 950 ° C

Since hot rolling needs to be terminated in an austenitic single-phase region in order to improve the resistance resistance crack resistance after annealing by homogenizing the microstructure of the steel sheet and reducing the anisotropy of the material, the finish rolling end temperature is 860 ° C or higher. do. On the other hand, when the finish rolling finish temperature is more than 950 ° C, the hot rolled structure becomes coarse and the grain size after annealing also coarsens, so the upper limit of the finish rolling finish temperature is set to 950 ° C.

[Cooling process after hot rolling]

Primary cooling process: cooling to 700 ° C or less at a first average cooling rate of 70 ° C / s or more

Although austenite transforms ferrite in the cooling process after the end of hot rolling, the ferrite coarsens at a high temperature, so that after the end of hot rolling, quenching makes the structure as homogeneous as possible and suppresses the formation of Ti-based precipitates. Therefore, first, as primary cooling, it cools to 700 degrees C or less at the 1st average cooling rate of 70 degrees C / s or more. When the first average cooling rate is less than 70 ° C / s, the ferrite is coarsened, so that the microstructure of the hot-rolled steel sheet becomes uneven, resulting in a decrease in delayed fracture characteristics and cross tensile strength after resistance spot welding. On the other hand, when the cooling stop temperature in primary cooling exceeds 700 degreeC, pearlite will generate | occur | produce excessively in the microstructure of a hot rolled steel sheet, and the final steel plate structure will become non-uniform, and eventually a crosslink after a delayed fracture characteristic and resistance spot welding is carried out. Tensile strength decreases.

Moreover, the cooling stop temperature of primary cooling shall be 700 degreeC or less, and shall be 500 degreeC or more.

Secondary cooling process: cooling to 450 ° C. or lower at a second average cooling rate of 5 to 50 ° C./s

When the average cooling rate in this secondary cooling is less than 5 degree-C / s, the ferrite or pearlite will generate | occur | produce excessively in the microstructure of a hot rolled steel sheet, the microstructure of a final steel plate will become heterogeneous, and a Ti type precipitate will also be coarse. In order to communicate with each other, delayed fracture resistance and cross tensile strength after resistance spot welding decrease. On the other hand, when the average cooling rate in secondary cooling exceeds 50 degree-C / s, since pearlite is produced | generated excessively in the microstructure of a hot rolled steel sheet, the element distribution of C will become nonuniform and the delayed fracture characteristic after hot press, and The cross tensile strength after resistance spot welding falls. In addition, in the cooling up to a temperature above 450 ° C, ferrite or pearlite is excessively generated in the microstructure of the hot rolled steel sheet, and coarse Ti-based precipitates are coarsened, resulting in delayed fracture resistance and cross tensile strength after resistance spot welding. Lowers.

Odor temperature: 450 ℃ or less

When the coiling temperature is higher than 450 ° C., ferrite and pearlite are excessively generated in the microstructure of the hot rolled steel sheet, the microstructure of the final steel sheet becomes uneven, and the delayed fracture resistance and the cross tensile strength after the resistance spot welding decrease. To avoid this, it is important to wind the bainite single phase. Moreover, when winding up at high temperature, Ti type precipitate will coarsen and the delayed fracture characteristic will fall. Therefore, in this invention, the upper limit of the winding temperature was 450 degreeC. Preferably it is 420 degrees C or less. The lower limit of the coiling temperature is not particularly defined, but when the coiling temperature is too low, hard martensite is excessively generated and the cold rolling load is increased, so 300 ° C or more is preferable.

[Pickling process]

After a hot rolling process, pickling is performed and the scale of a hot rolled sheet surface layer is removed. This pickling treatment is not particularly limited and may be performed according to a conventional method.

[Cold rolling process]

The cold rolling process of rolling with the cold rolled sheet of predetermined plate | board thickness is performed. This cold rolling step is not particularly limited and may be carried out in accordance with a conventional method.

[Annealing step]

After cold rolling, it heats to the temperature range of 700-830 degreeC at the average temperature increase rate of 5-20 degreeC / s, and it cracks for 15 second or more and 600 second in the temperature range.

This annealing is performed in order to advance the recrystallization after cold rolling, and to control the microstructure of the member after a hot press, and the distribution state of Ti type precipitate.

In this annealing step, the recrystallization hardly proceeds when heated too rapidly, so the upper limit of the average temperature increase rate is set to 20 ° C / s. On the other hand, if the temperature increase rate is too small, ferrite and martensite grains coarsen and a desired microstructure cannot be obtained after hot pressing, so an average temperature increase rate of 5 ° C / s or more is required. Preferably it is 8 degree-C / s or more. By controlling this average temperature increase rate, refinement | miniaturization of a crystal grain is attained.

And it heats to the crack temperature range of 700-830 degreeC mentioned later.

Crack temperature: 700 ~ 830 ℃

The crack temperature is taken as the temperature range of the two phase region of ferrite and austenite. If it is less than 700 degreeC, martensite fraction will become small and high concentration of C and Mn will concentrate in austenite, and the desired cementite precipitation state will not be obtained after hot press. Therefore, the lower limit of the cracking temperature is 700 ° C. On the other hand, when the cracking temperature is too high, grain growth of austenite becomes remarkable, coarsening of crystal grains and Ti-based precipitates and deterioration in delayed fracture characteristics decreases the cracking temperature to 830 ° C or lower. Preferably it is 810 degrees C or less.

Crack holding time: 15 to 600 seconds

At the above crack temperature, it is necessary to maintain at least 15 s for the progress of recrystallization and the austenite transformation of some or all of the tissues. On the other hand, when the holding time is excessively long, micro segregation of Mn is encouraged and bending workability deteriorates, so the holding time is preferably within 600 seconds.

[Cooling process]

Cooling conditions after cracking: cooling to a temperature range of 600 ° C or lower at a third average cooling rate of 5 ° C / s or higher

After the said cracking process (annealing process), it is necessary to cool from a crack temperature to the temperature range (cooling stop temperature) of 600 degrees C or less at the average cooling rate of 5 degrees C / s or more. If the average cooling rate is less than 5 ° C / s, ferrite transformation proceeds during cooling, the volume fraction of martensite in the cold rolled steel sheet decreases, and the Ti-based precipitate coarsens, making it difficult to secure delayed fracture resistance. . Although it does not specifically define about the upper limit of this average cooling rate, 30 degreeC / s or less is preferable from a viewpoint of an installation and a cost. Moreover, when a cooling stop temperature exceeds 600 degreeC, a pearlite is produced | generated excessively and since the predetermined volume ratio in the microstructure of a steel plate cannot be obtained, delayed-destructive fracture characteristic falls.

In the series of manufacturing processes described above, it is particularly important in the present invention that the continuous casting step, the hot rolling step (including subsequent primary and secondary cooling steps), and the annealing step after cold rolling (the third step thereafter) Cooling process).

That is, by appropriately controlling the above-described continuous casting process, hot rolling process and annealing process, crystal grains are refined, Mn segregation is eliminated, and the distribution state of Ti-based precipitates is improved, resulting in an average grain size of 4 µm or less. Ti-based precipitates containing Zite in a volume ratio of 5 to 45% and having a particle diameter of less than 0.10 m in a range from the steel plate surface to a plate thickness direction of 100 m in a direction parallel to the plate thickness direction of the steel sheet. Microstructures can be obtained, with more than 15 being present on average per ^two .

Thereafter, plating treatment such as hot dip galvanizing may be performed, or may be used while being cold rolled steel sheet without performing such plating treatment.

[Plating process]

Although the cold-rolled steel sheet for hot press of this invention may be used as it is, the cold-rolled steel sheet manufactured by the manufacturing process mentioned above, Al-based plating process or Zn-based plating for forming an Al type plating layer or a Zn type plating layer according to the objective. You may perform a process.

Such plating treatment is not limited at all, and any known hot dip plating method, electroplating method, vapor deposition plating method, or the like can be applied. Moreover, you may perform an alloying process after plating process. As a typical plating treatment, the Al-based plating treatment may be performed by performing molten aluminum (Al) plating or molten Al-Si plating, and the Zn-based plating treatment may be performed by hot dip galvanizing or electro zinc plating. The treatment or an alloying treatment further after hot dip galvanizing is mentioned.

Further, temper rolling may be performed on the cold rolled steel sheet. Preferable elongation at this time is 0.05 to 2.0%.

The hot rolled steel sheet obtained as described above is subjected to hot press to form a hot press member. However, the hot press method at this time is not particularly limited and may be performed according to a conventional method. Although an example is shown below, it is not limited to this.

For example, the material of the cold-rolled steel sheet for hot press, an electric furnace, the gas, an electrically conductive heating, using, for example, far-infrared heating, Ac ₃ heated to a temperature of transformation point ~ 1000 ℃, and 0 in this temperature range ~ After hold | maintaining for 600 second, a steel plate is conveyed with a press machine, and what is necessary is just to hot-press in the range of 550-800 degreeC. The temperature increase rate at the time of heating the cold-rolled steel sheet for hot presses may be 3-200 degreeC / s.

Here, Ac ₃ transformation point can be calculated | required by the following formula.

Ac ₃ transformation point (℃) = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu + 41Mo

However, the element symbol in a formula shows content (mass%) of each element. About the element which does not contain, it calculates as 0.

Example

EMBODIMENT OF THE INVENTION Hereinafter, the Example of this invention is described.

In addition, this invention is not limited by the Example described below originally, It is also possible to change suitably and to implement in the range which may be suitable for the meaning of this invention, and all of them are this invention. It is included in the technical scope.

After melting the steel of the component composition shown in Table 1, and making it into the slab by continuous casting on the conditions shown in Table 2, after heating at 1250 degreeC, hot rolling is performed on the conditions shown in Table 2 of finishing rolling end temperature (FDT). Was carried out. Subsequently, after cooling the hot-rolled steel sheet to the cooling stop temperature (first cooling temperature) at the 1st average cooling rate (cold speed 1) shown in Table 2, to a coiling temperature (CT) at a 2nd average cooling temperature (cold speed 2). It cooled and wound up in the coil. In addition, about some samples, after hot-rolling, it cooled at constant speed and wound up in the coil, without performing two stages of cooling processes.

Subsequently, the obtained hot rolled sheet was cold-rolled at the reduction ratio shown in Table 2 after pickling, and it was set as the cold rolled sheet (plate thickness: 1.4 mm).

Subsequently, the cold-rolled steel sheet thus obtained is subjected to annealing treatment under the conditions shown in Table 2 in the continuous annealing line (CAL) or the continuous hot dip plating line (CGL), and the cold rolled steel sheet (CR) is used for the steel sheet which has passed through CAL. About the steel plate which passed CGL, the hot-dip galvanized steel plate (GI) was obtained. Moreover, about the part of the steel plate which passed CGL, after carrying out a hot dip galvanizing process, the alloying process was further performed at 550 degreeC, and the alloying hot dip galvanized steel sheet (GA) was obtained. Moreover, the molten aluminum plating process was performed and the molten aluminum plating steel plate (AS) was obtained. In addition, after annealing partly in CAL, the electrogalvanized steel plate (EZN) was obtained in the electrogalvanization line (EGL).

Next, about the obtained cold rolled steel sheet (including the plated steel plate), hot pressing was performed on the conditions shown in Table 3.

The metal mold | die used by the hot press is 70 mm of punch width, punch shoulder R 4 mm, die shoulder R 4 mm, and forming depth is 30 mm. The heating on the cold rolled steel sheet was performed in the atmosphere using either an infrared heating furnace or an atmosphere heating furnace depending on the heating rate. In addition, the cooling after press was performed combining the insertion between the punch and die of the steel plate, and the air cooling on the die opened from the insertion, and cooled to 150 degreeC from the press (starting) temperature. At this time, the cooling rate was adjusted by changing the time to hold a punch at bottom dead center in the range of 1 to 60 second.

The JIS No. 5 tensile test piece was extract | collected from the position of the hat bottom of the hot press member obtained in this way, the tensile test was performed based on JISZ22241, and the tensile strength (TS) was measured.

Moreover, about the test of a delayed fracture characteristic, the JIS No. 5 tensile test piece was extract | collected from the position of the hat bottom of a hot press member, and the static load test was done. The load was applied while immersing in a solution of hydrochloric acid (pH = 3.0) at room temperature to evaluate the presence or absence of breakage. The load stress is 900 MPa and 1200 MPa. When both load stresses do not break for 100 hours or more, the delayed fracture resistance is good (○), but the load stress 900 MPa does not break for 100 hours or more, but the load stress is 1200 MPa. In the case of breaking in less than 100 hours in the case of rupture in less than 100 hours in both suitable (Δ) and both load stresses, the delayed fracture resistance was regarded as inferior (×).

The cross tensile strength after resistance spot welding cut out the 50 * 150 mm cross tensile test piece based on the cross tension test method (JIS Z 3137), and used the test piece which carried out resistance welding. The conditions of resistance welding performed the resistance spot welding using the single-phase direct current (50 Hz) resistance welding machine by the servomotor pressurization mounted on the welding gun, and produced the tensile test piece which has a resistance welding part. The pair of electrode chips used was a DR type electrode of alumina dispersed copper having a radius of curvature R 40 mm at the tip and a tip diameter of 6 mm. As for welding conditions, welding force was adjusted so that press force was 4500 N, energization time was 19 cycles (50 Hz), hold time was 1 cycle (50 Hz), and nugget diameter was 6.1 mm. When the cross tensile strength is 6.5 kN or more, the cross tensile strength after resistance spot welding is good (○), when 5 kN or more is suitable (△), and when less than 5 kN, the cross tensile strength after resistance spot welding is inferior (×). It was.

The volume fraction of the martensite of the cold rolled steel sheet after annealing and the member after hot pressing is corroded with 3 vol% nital after polishing a cross section parallel to the rolling direction and parallel to the thickness direction of the steel sheet, followed by SEM (scanning electron microscope). ) Was observed at a magnification of 5000 times, and the area ratio was measured by the point count method (based on ASTM E562-83 (1988)), and the area ratio was defined as the volume ratio. The average grain size of martensite, guosteneite, and ferrite was obtained from a steel sheet microstructure photograph (image taken at a magnification of 5000 times of 10 μm × 20 μm at 10 points) using Image Cyber-Pro from Media Cybernetics. The area of each crystal grain can be computed by acquiring the photograph which identified each crystal grain of each austenite, ferrite, and martensite beforehand, the circle equivalent diameter was computed, and those values were averaged and calculated | required.

The Ti-based precipitates and cementite have a particle diameter of 0.5 μm × 0.5 μm at a magnification of 10000 times using a TEM (transmission electron microscope) for a cross section parallel to the thickness direction of both the cold rolled steel sheet and the press member. Ten points were observed and the particle size was calculated | required by making a minimum into 0.005 micrometer and calculating the equivalent circular diameter using Image-Pro by Media Cybernetics. The number of Ti-based precipitates having a particle diameter of less than 0.10 μm and cementite having a particle size of 0.05 μm or more was observed using a TEM (transmission electron microscope) at a 10 × magnification of 10 viewing points of 0.5 μm × 0.5 μm, and the average of 10 points The number density was calculated. In this method, a Ti-based precipitate having a particle size of 0.005 µm or more could be counted.

Table 4 shows the microstructures of the thus obtained cold rolled steel sheet and hot press member. In addition, Table 5 shows the measurement results of tensile properties, delayed fracture resistance, and cross tensile strength of the hot press member after resistance spot welding.

TABLE 1

TABLE 2-1

Table 2-2

TABLE 2-3

Table 3-1

Table 3-2

Table 3-3

Table 4-1

Table 4-2

Table 4-3

Table 5-1

Table 5-2

Table 5-3

As shown in Table 5, all of the invention examples in which the microstructure of the component after composition and the hot press satisfy the appropriate range of the present invention need not only mention high tensile strength, but also excellent delayed fracture resistance and high resistance spot. The cross tensile strength after welding was obtained together.

Claims (13)

The steel chemical component of the member is, by mass%, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.1% or more and 2.4% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01 % Or more, 0.50% or less, N: 0.010% or less, Ti: 0.005% or more, 0.15% or less, and furthermore, one or two kinds selected from Mo: 0.50% or less and Cr: 0.50% or less, and the balance is Consisting of Fe and inevitable impurities,
In the microstructure of the member, cementite having an austenite average grain size of 8 µm or less, a volume fraction of martensite of 90% or more, and a particle diameter of 0.05 µm or more was averaged per 200 µm ^{2 of} a cross section parallel to the thickness direction of the member. More than,
Further, at least 10 Ti-based precipitates having a particle diameter of less than 0.10 μm in the range from the surface of the member to 100 μm in average per 100 μm ^{2 of} the cross section parallel to the thickness direction of the member and having a tensile strength of at least 1780 MPa were hot Press member. The method of claim 1,
The said member is further in mass% Nb: 0.15% or less, B: 0.0050% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less, V : 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less and W: 0.10 Hot press member containing 1 type (s) or 2 or more types chosen from% or less. The method according to claim 1 or 2,
Among the strong chemical components of the member, in particular, C, P, Mn, Cr, Mo and Ti, the hot press member that satisfies the following formula (1).
(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)
Here, [M] is content (mass%) of M element, and when it does not contain element [M], it calculates as 0. The method according to any one of claims 1 to 3,
The hot press member which has an Al type plating layer or a Zn type plating layer in the surface layer of the said member. The chemical composition of the steel sheet is, in mass%, C: 0.28% or more and less than 0.42%, Si: 1.5% or less, Mn: 1.1% or more and 2.4% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% 0.50% or more, N: 0.010% or less, Ti: 0.005% or more, 0.15% or less, and further, one or two kinds selected from Mo: 0.50% or less and Cr: 0.50% or less, and the balance is Fe. And inevitable impurities,
The microstructure of the steel sheet contains a martensite having an average grain size of 4 µm or less in a range of 5 to 45% by volume ratio, and a Ti system having a particle diameter of less than 0.10 µm in a range from the surface of the steel sheet to 100 µm in the plate thickness direction. The cold-rolled steel sheet for hot press in which 15 or more precipitates exist in an average per 100 micrometer <^2> of cross sections parallel to the plate thickness direction of a steel plate. The method of claim 5, wherein
The steel sheet is, in mass%, Nb: 0.15% or less, B: 0.0050% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less, V : 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less and W: 0.10 Cold rolled steel sheet for hot press containing 1 type (s) or 2 or more types selected from% or less. The method according to claim 5 or 6,
Among the chemical components of the steel sheet, in particular, C, P, Mn, Cr, Mo and Ti is a cold rolled steel sheet for hot pressing, the following formula (1).
(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)
Here, [M] is content (mass%) of M element, and when it does not contain element [M], it calculates as 0. The method according to any one of claims 5 to 7,
The cold rolled steel sheet for hot pressing of which the said steel plate has an Al type plating layer or a Zn type plating layer on the surface. As a method of manufacturing the cold-rolled steel sheet for hot pressing according to claim 5,
In mass%, C: 0.28% or more but less than 0.42%, Si: 1.5% or less, Mn: 1.1% or more and 2.4% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.50% or less, N : 0.010% or less, Ti: 0.005% or more, 0.15% or less, and 1 or 2 types selected from Mo: 0.50% or less and Cr: 0.50% or less, and remainder consists of Fe and an unavoidable impurity. Molten steel is continuously cast to a slab, and the slab is cooled to 650 ° C. within 6 h,
Then, it reheats, the rolling reduction rate of the final pass of finishing rolling is made into 12% or more, and the rolling reduction rate of the pass just before the last pass is made into 15% or more, and hot rolling is carried out on the conditions of finishing rolling completion temperature of 860-950 degreeC. ,
After the said hot rolling, the 1st average cooling rate to a cooling stop temperature shall be 70 degreeC / s or more, and the primary cooling which cools to the cooling stop temperature of 700 degrees C or less is performed,
After the said primary cooling, the 2nd average cooling rate to the winding temperature shall be 5-50 degreeC / s, the secondary cooling which winds up at the winding temperature of 450 degrees C or less is performed,
Next, after pickling the wound hot rolled steel sheet and performing cold rolling, it heats to the temperature range of 700-830 degreeC at the average temperature increase rate of 5-20 degreeC / s, and the annealing which cracks for 15 to 600 second in the temperature range is carried out. Conduct,
After the said cracking process, the 3rd average cooling rate shall be 5 degrees C / s or more, and the 3rd cooling which cools to the cooling stop temperature of 600 degrees C or less is performed, The manufacturing method of the cold rolled steel sheet for hot presses. The method of claim 9,
The molten steel is, in mass%, further Nb: 0.15% or less, B: 0.0050% or less, Sb: 0.001% or more and 0.020% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less, V : 0.15% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.50% or less, Zn: 0.10% or less, Co: 0.10% or less, Zr: 0.10% or less, Ta: 0.10% or less and W: 0.10 The manufacturing method of the cold rolled steel sheet for hot press containing 1 type (s) or 2 or more types chosen from% or less. The method according to claim 9 or 10,
Among the chemical components of molten steel, in particular, a method for producing a cold rolled steel sheet for hot pressing in which C, P, Mn, Cr, Mo, and Ti satisfy the following formula (1).
(6 [C] + 2 [Mn] + 49 [P]) / ([Cr] / 2 + [Mo] / 3 + 7 [Ti]) ≤ 30.5 ... (1)
Here, [M] is content (mass%) of M element, and when it does not contain element [M], it calculates as 0. The method according to any one of claims 9 to 11,
A method of producing a cold rolled steel sheet for hot pressing, wherein after the third cooling, an Al-based plating treatment or a Zn-based plating treatment is further applied to the steel sheet surface. Claim 5 to process for producing a hot press member to the eighth hot press forming cold-rolled steel sheet according to any one of claims, carried out after heating, hot press at a temperature range of Ac ₃ transformation point ~ 1000 ℃.